Because in our experience, migrating from Windows 2008R2 to a new Windows 2012R2 server caused all our tasks that use network resources to fail. I've spend the whole afternoon trying to get this working, deleting and re-creating credentials and they still fail.
There is something different when running on Windows 2012.
Hopefully, there will be a fix soon. Until then we will have to keep running on 2008. We do have over 1000 tasks in 200 groups, maybe its a capacity issue. We couldn't upgrade to 7.7.7 on the old server, it wouldn't process the import and crashed.
What would be helpful is a proper explanation of the credentials settings and a way to get better logs when a job runs and either does nothing or returns a error number I'v reported in other posts.
I suspect this is related to the way VisualCron scripts the execution of the task internally and is not passing back the internal error codes/messages. It's proving very hard to debug,